PTY(4) BSD Programmer's Manual PTY(4)
pty - pseudo terminal driver
pseudo-device pty [count]
[count]
The pty driver provides support for a device-pair termed a pseudo
terminal. A pseudo terminal is a pair of character devices, a master dev-
ice and a slave device. The slave device provides to a process an inter-
face identical to that described in tty(4). However, whereas all other
devices which provide the interface described in tty(4) have a hardware
device of some sort behind them, the slave device has, instead, another
process manipulating it through the master half of the pseudo terminal.
That is, anything written on the master device is given to the slave dev-
ice as input and anything written on the slave device is presented as in-
put on the master device.
In configuring, if an optional count is given in the specification, space
for that number of pseudo terminal pairs is preallocated. If the count is
missing or is less than 2, a default count of 8 is used. This is not a
hard limit--space for additional pseudo terminal pairs is allocated on
demand up to the limit imposed by the kern.tty.maxptys sysctl(8) (992 by
default).
The following ioctl(2) calls apply only to pseudo terminals:
TIOCSTOP Stops output to a terminal (e.g., like typing '^S'). Takes no
parameter.
TIOCSTART Restarts output (stopped by TIOCSTOP or by typing '^S').
Takes no parameter.
TIOCPKT Enable/disable packet mode. Packet mode is enabled by speci-
fying (by reference) a non-zero parameter and disabled by
specifying (by reference) a zero parameter. When applied to
the master side of a pseudo terminal, each subsequent read(2)
from the terminal will return data written on the slave part
of the pseudo terminal preceded by a zero byte (symbolically
defined as TIOCPKT_DATA), or a single byte reflecting control
status information. In the latter case, the byte is an
inclusive-or of zero or more of the bits:
TIOCPKT_FLUSHREAD whenever the read queue for the terminal
is flushed.
TIOCPKT_FLUSHWRITE whenever the write queue for the terminal
is flushed.
TIOCPKT_STOP whenever output to the terminal is
stopped a la '^S'.
TIOCPKT_START whenever output to the terminal is res-
tarted.
TIOCPKT_DOSTOP whenever t_stopc is '^S' and t_startc is
'^Q'.
TIOCPKT_NOSTOP whenever the start and stop characters
are not '^S/^Q'.
While this mode is in use, the presence
of control status information to be read
from the master side may be detected by a
select(2) for exceptional conditions.
This mode is used by rlogin and rlogind
to implement a remote-echoed, locally
'^S/^Q' flow-controlled remote login with
proper back-flushing of output; it can be
used by other similar programs.
TIOCPKT_IOCTL When this bit is set, the slave has
changed the termios(4) structure (TTY
state), and the remainder of the data
read from the master side of the pty is a
copy of the new termios(4) structure.
This is used by telnetd(8) to implement
TELNET "line mode" - it allows the
telnetd(8) to detect tty(4) state changes
by the slave, and negotiate the appropri-
ate TELNET protocol equivalents with the
remote peer.
TIOCUCNTL Enable/disable a mode that allows a small number of simple
user ioctl(2) commands to be passed through the pseudo termi-
nal, using a protocol similar to that of TIOCPKT. The
TIOCUCNTL and TIOCPKT modes are mutually exclusive. This mode
is enabled from the master side of a pseudo terminal by
specifying (by reference) a nonzero parameter and disabled by
specifying (by reference) a zero parameter. Each subsequent
read(2) from the master side will return data written on the
slave part of the pseudo terminal preceded by a zero byte, or
a single byte reflecting a user control operation on the
slave side. A user control command consists of a special
ioctl(2) operation with no data; the command is given as
UIOCCMD(n), where n is a number in the range 1-255. The
operation value n will be received as a single byte on the
next read(2) from the master side. The ioctl(2) UIOCCMD(0) is
a no-op that may be used to probe for the existence of this
facility. As with TIOCPKT mode, command operations may be
detected with a select(2) for exceptional conditions.
TIOCREMOTE A mode for the master half of a pseudo terminal, independent
of TIOCPKT. This mode causes input to the pseudo terminal to
be flow controlled and not input edited (regardless of the
terminal mode). Each write to the control terminal produces a
record boundary for the process reading the terminal. In nor-
mal usage, a write of data is like the data typed as a line
on the terminal; a write of 0 bytes is like typing an end-of-
file character. TIOCREMOTE can be used when doing remote line
editing in a window manager, or whenever flow controlled in-
put is required.
The standard way to allocate pty devices is through openpty(3), a func-
tion which internally uses a PTMGET ioctl(2) call on the /dev/ptm device.
The PTMGET command allocates a free pseudo terminal, changes its owner-
ship to the caller, revokes the access privileges for all previous users,
opens the file descriptors for the master and slave devices and returns
them to the caller in struct ptmget.
struct ptmget {
int cfd;
int sfd;
char cn[16];
char sn[16];
};
The cfd and sfd fields are the file descriptors for the controlling and
slave terminals. The cn and sn fields are the file names of the control-
ling and slave devices.
/dev/pty[p-zP-T][0-9a-f] master pseudo terminals
/dev/tty[p-zP-T][0-9a-f] slave pseudo terminals
/dev/ptm pseudo terminal management device
openpty(3), tty(4), sysctl(8)
The pty driver appeared in 4.2BSD. The /dev/ptm device was added in
OpenBSD 3.5.
The ptm device will only work on systems where the /dev directory has
been properly populated with pty(4) device nodes following the naming
convention used in OpenBSD. Since ptm impersonates the super user for
some operations it needs to perform to complete the allocation of a pseu-
do terminal, the /dev directory must also be writeable by the super user.
MirBSD #10-current November 30, 1993 2